"A good stock of examples, as large as possible, is indispensable for a thorough understanding of any concept, and when I want to learn something new, I make it my first job to build one." – Paul Halmos

Posts Tagged ‘Lawvere theories’

Let be a set with two elements. The category of Boolean functions is the category whose objects are the finite powers of and whose morphisms are all functions between these sets. For a computer scientist, the morphisms of this category have the interpretation of functions which input and output finite sequences of bits.

Since this category has finite products and is freely generated under finite products by a single object, namely , it is a Lawvere theory.

Often in mathematics we define constructions outputting objects which a priori have a certain amount of structure but which end up having more structure than is immediately obvious. For example:

Given a Lie group , its tangent space at the identity is a priori a vector space, but it ends up having the structure of a Lie algebra.

Given a space , its cohomology is a priori a graded abelian group, but it ends up having the structure of a graded ring.

Given a space , its cohomology over is a priori a graded abelian group (or a graded ring, once you make the above discovery), but it ends up having the structure of a module over the mod-Steenrod algebra.

The following question suggests itself: given a construction which we believe to output objects having a certain amount of structure, can we show that in some sense there is no extra structure to be found? For example, can we rule out the possibility that the tangent space to the identity of a Lie group has some mysterious natural trilinear operation that cannot be built out of the Lie bracket?

In this post we will answer this question for the homotopy groups of a space: that is, we will show that, in a suitable sense, each individual homotopy group is “only a group” and does not carry any additional structure. (This is not true about the collection of homotopy groups considered together: there are additional operations here like the Whitehead product.)

Previously we looked at several examples of -ary operations on concrete categories . In every example except two, was a representable functor and had finite coproducts, which made determining the -ary operations straightforward using the Yoneda lemma. The two examples where was not representable were commutative Banach algebras and commutative C*-algebras, and it is possible to construct many others. Without representability we can’t apply the Yoneda lemma, so it’s unclear how to determine the operations in these cases.

However, for both commutative Banach algebras and commutative C*-algebras, and in many other cases, there is a sense in which a sequence of objects approximates what the representing object of “ought” to be, except that it does not quite exist in the category itself. These objects will turn out to define a pro-object in , and when is pro-representable in the sense that it’s described by a pro-object, we’ll attempt to describe -ary operations in terms of the pro-representing object.

Groups are in particular sets equipped with two operations: a binary operation (the group operation) and a unary operation (inverse) . Using these two operations, we can build up many other operations, such as the ternary operation , and the axioms governing groups become rules for deciding when two expressions describe the same operation (see, for example, this previous post).

When we think of groups as objects of the category , where do these operations go? They’re certainly not morphisms in the corresponding categories: instead, the morphisms are supposed to preserve these operations. But can we recover the operations themselves?

It turns out that the answer is yes. The rest of this post will describe a general categorical definition of -ary operation and meander through some interesting examples. After discussing the general notion of a Lawvere theory, we will then prove a reconstruction theorem and then make a few additional comments.

Here’s what seems like a silly question: what’s the universal group? That is, what’s the universal example of a set together with maps

satisfying the identities

,

,

?

A moment’s reflection shows that there isn’t such a group; the existence of the groups , where is an arbitrary set, shows that there exist groups of arbitrarily large cardinality, so no particular group can be universal.